Speaking from inside Madaya, a doctor working with the Syrian American Medical Society said deaths had been occurring almost daily due to a lack of food.

The doctor Khaled, who would only be identified by his first name, told the ABC that babies were being given salted water because there is no milk.

Video purportedly filmed inside the town and sent to the ABC shows a severely malnourished baby and small boy.

The skinny child is heard saying he has not eaten properly for seven days.

"We want doctors to help, the UN should get involved to see how people are starving here, we must get some help, food with in the next 24 hours," Dr Khaled said.

"Every day [that] passes by it costs us lives of people. Please, the international community must act fast, please act fast to save the lives of the people here."

A senior official with Medecins San Frontiers confirmed to the ABC that they believe people in Madaya are already dying of starvation and that if the siege is not lifted in the coming weeks, they expect more deaths to be recorded.

"We are very worried about recording more deaths due to malnutrition or lack of access to food," the official said, speaking on anonymity because they work inside besieged parts of Syria.

'Madaya has become like a large prison'

Residents inside Madaya report that people who have tried to break the siege have been shot and killed while trying to escape.

Landmines have also reportedly been laid around the town to prevent people from leaving.